1. Field of the Invention
The present invention relates to a half-tone reproduction method and a half-tone reproduction apparatus for reproducing a half-tone image with a thermal printer and other printers, and, more particularly, to a half-tone reproduction method and a half-tone reproduction apparatus for reproducing a half-tone image using a multi-level Dither matrix.
2. Description of the Related Art
A Dither method and fixed pattern method are known as methods which reproduce a pseudo half-tone using a recording device, such as a thermal printer or a laser beam printer, suitable for reproduction of a binary image. Of these, the Dither method is often used as a system for expressing a pseudo half-tone since it can provide a fine gradation reproduction of an image part with s smooth density change without degrading the resolution of an image part with a relatively sharp density change. According to the Dither method, an input density level is compared with each threshold level of a Dither matrix, and when the input density level is greater than the threshold level, dots are formed, and when it is smaller, no dots are formed. Therefore, the gradation of a image with a smooth change in density level is expressed by the overall matrix by modulating the number of recording dots (density) within the matrix, while an edge part of an image with a sharp change in density level is finely reproduced at the level of each dot in the matrix. Since the Dither method provides fine reproduction of a smooth change in density, the greater the number of gradations, the greater the matrix, thus reducing the resolution accordingly; on the other hand, the smaller the matrix, the higher the resolution with reduction in the number of gradations.
Another type of Dither method is known that utilizes a threshold-level matrix in which multi-levels are assigned to a single dot in order to express a large number of gradations with a small matrix. (This particular method will be hereinafter called a multi-level Dither method, and is disclosed in Japanese Patent Disclosure 61-30793.) This Japanese document discloses the following two rules for varying the energy control level with a change in input density level. The first rule is such that the level of only a specific dot is varied, and when it reaches the lowest or highest level, then a dot to be varied is changed to another one. The other one is such that the levels of the individual dots within the matrix are varied so that their overall levels become even. As example of the latter rule applied particularly to a lesser beam printer, a four-level Dither method has been reported by Kawamura et al. in Proceedings of Non-impact Printing Technologies Symposium, 4-5, p. 94-99, 1984 July 24. Another Japanese Patent Disclosure 60-240277 discloses a different rule than the mentioned two, which involves a mixture of dots whose control levels increase and those whose control levels decrease at the time the density level is changed.
However, such conventional multi-level Dither methods would occur gradation jumping or deformation an image at a high density region, and thus would not provide a fine gradation expression, nor would they improve the resolution, to the expected level.
This problem will be explained below with reference to thermal transfer recording. As the amount of supplied energy to an isolated pattern consisting of a single dot is gradually varied, the amount of ink to be transferred onto a recording sheet almost monotonously increases. After ink transfer for one dot is completed, such monotonous increase is established. At a level where isolated dots are no yet formed, however, the uncertainty as to whether or not the isolated dots will be transferred would cause instability. In the case where isolated dots are formed adjacent to one another, there would be an unstable region formed between the dots by the coupling or separation of the dots. Further, in a high density region, that part which should be in white may be smeared in black, resulting in the instability too.
From the above, it should be understood that there exist some instabilities in the relationship between the input energy of the individual heat generating elements and the print density between low and high density regions. Such instabilities could cause gradation jumping or deformation at a high density region, thus deteriorating the quality of an attained image.